Integrated circuit microprocessors are widely used for a variety of digital data processing applications. In general, most conventional microprocessors comprise an arithmetic/logic unit (ALU), a plurality of registers, and a memory address and data bus which may or may not be driven externally from the chip. Program instructions read from the memory are decoded into one or more microinstructions, the microinstructions operating to control the internal logic of the microprocessor to accomplish a predefined action as specified by the instruction. For example, an ADD instruction is typically decoded into a plurality of microinstructions which perform the task of accessing the data to be added, directing the data into the ALU and storing the result of the addition. In conventional microprocessors these microinstructions are stored internally within a control store which is typically a read only memory which is integrated within the microprocessor circuitry. This creates a problem in that the control store is normally inaccessible to devices outside of the microprocessor, making the reading and/or modification of the microinstructions within the control store by external devices difficult or impossible.
One such external device may be a coprocessor, such as a floating point arithmetic acceleration unit. Such acceleration units typically work in conjunction with the microprocessor and provide extended mathematical capability in those applications where such extended capability is required. As can be appreciated, the degree of coupling between the microprocessor and the coprocessor is an important system consideration in that the degree of coupling typically affects the speed and efficiency of the execution of a given program.
Other factors which influence the speed and efficiency of conventional microprocessors are the flexibility of various internal registers, such as a status register which has bits reflecting the results of various logical, arithmetic and other conditions within the device. The ability to set, reset and modify the state of the status bits in order that conditional branch instructions may be employed is thus an important aspect in the overall efficiency and instruction processing capability of the microprocessor.
Furthermore, for those microprocessors which employ some type of memory management unit (MMU) for translating virtual machine addresses to a physical memory address the efficiency of the MMU in maintaining an address translation table which ensures a high "hit" rate for translated instructions is an important system consideration.